With concerns about rising fuel costs, energy security, statewide brownouts, and demand surges that exceed electrical supply, solar electric systems are needed to meet a greater share of energy needs. Photovoltaic devices, i.e., solar cells, are capable of converting solar radiation into usable electrical energy. The energy conversion occurs as the result of what is known as the photovoltaic effect. Solar radiation impinging on a solar cell and absorbed by an active region of semiconductor material generates electricity.
In recent years, technologies relating to thin-film solar cells have been advanced to realize inexpensive and lightweight solar cells and, therefore, thinner solar cells manufactured with less material have been demanded. This is especially true in the space industry with the solar cells powering satellites and other space vehicles.
The current state of the art in solar cell design is to deposit a photoactive material onto a substrate. Hydrogenated amorphous silicon-germanium (a-SiGe:H) alloy accounts for over half the materials in most commercial multi-junction amorphous silicon thin film solar cells. a-SiGe:H has been used in the tandem and triple-junction solar cells to improve the red response. However, a-SiGe:H alloy has poorer electronic properties than a-Si:H because of higher defect densities, weaker hydrogen bonds and other structural defects. This problem is more pronounced for low bandgap a-SiGe:H alloy with Ge content greater than 50%. In addition, the cost of germanium gas is high.
Various techniques have been tried to improve the property of the a-SiGe:H alloy. Growing a-SiGe:H alloy near the threshold of microcrystallinity using hydrogen dilution at low deposition rate (˜1 Å/s) by rf plasma-enhanced CVD (PECVD) and applied graded alloy layers were two of many techniques that have significantly improved a-SiGe:H solar cell performance. Despite the recent development of a microcrystalline silicon (μc-Si) solar cell and its potential of replacing a-SiGe:H materials, a-SiGe:H solar cell exhibits higher open circuit voltage (Voc), a tunable bandgap, and potential for further improvement. With these considerations, a-SiGe:H alloys are still considered as promising materials for use in commercial a-Si:H based solar cells fabrications.
Deposition rate is one of the important factors to increase the throughput and reduce the capital cost for PV production. The deposition rate of the photoactive material onto the substrate has been, typically, approximately one (1) Å/second or less with a typical ten (10%) percent stable efficiency for a-Si:H solar cells. It is even more crucial for a-SiGe:H because of the large amount of materials used in the solar cells. The best a-Si:H based solar cells are made at the deposition rate about 1 Å/s. Up to date, the properties of the high deposition rate (greater than 1 Å/s) materials remain inferior to the one at 1 Å/s. The efficiency of the high rate solar cells, as a consequence, is lower than the ones at 1 Å/s.
Accordingly, the time to manufacture the solar cell increases the cost of manufacture thereby increasing the cost to the ultimate user. In the very near future, to further increase the volume of production of solar cells to meet the high demand, an efficient high deposition rate will be required.
Accordingly, there exists a need for a thin-film solar cell fabricated with a high deposition rate. Additionally, a need exists for a thin-film solar cell with a high deposition rate and increased efficiency. Furthermore, there exists a need for a thin-film solar cell fabricated with a high deposition rate utilizing a hot wire chemical vapor deposition technique with optimum parameters to achieve efficient high deposition rates of approximately ten (10) Å/second.